Proof. Let α,β,γαβγ\alpha,\,\beta,\,\gamma be arbitrary elements of the image f⁢(G)fGf(G) and a,b,cabca,\,b,\,c some elements of GGG such that f⁢(a)=α,f⁢(b)=β,f⁢(c)=γformulae-sequencefaαformulae-sequencefbβfcγf(a)=\alpha,\,f(b)=\beta,\,f(c)=\gamma. Then